1. Field of the Invention
This invention relates to electronic theft detection and more particularly it concerns novel methods and apparatus for indicating the passageway through which protected goods are carried.
2. Description of the Prior Art
Prior art electronic theft detection systems of the type with which the present invention may be used are shown and described by way of example in U.S. Pat. No. 3,500,373 and No. 4,118,693.
In general, these prior systems make use of a transmitter antenna and a receiver antenna spaced apart from each other to define a passageway, usually at a doorway or other limited egress, through which a protected article may be carried. The protected article is provided with a "target" comprising a special electronic element or circuit capable of producing a characteristic electromagnetic signal in response to an electromagnetic field incident upon it. A transmitter is connected to the transmitter antenna and causes it to generate an interrogating electromagnetic field in the passageway. A receiver is connected to the receiver antenna and is designed to produce an alarm signal when a characteristic electromagnetic signal from a target in the passageway is received at the receiver antenna. The alarm signal is then used to activate a visual or acoustical alarm.
In some applications it is desirable to provide closely spaced passageways through which protected articles may be carried. A problem which arises in such cases is that the interaction of a target with interrogating fields in one passageway may cause electromagnetic signals to be produced and detected in adjacent passageways. The prior art discloses a technique for overcoming this problem. More specifically, as disclosed in U.S. Pat. No. 4,135,184, there are provided a plurality of individual, but closely spaced theft detection systems, each comprising its own set of transmitter and receiver antennas and associated transmitter, receiver and alarm. The antennas of each set extend horizontally and are vertically spaced apart from each other, with one on the floor and the other overhead of the passageway. Each system is provided with a multiplexing arrangement which permits only one system to be in operation at any one time. By switching the multiplexing arrangement very rapidly the various passageways are interrogated in succession and it becomes possible to ascertain the particular passageway through which a target is being carried.
The multiplexing technique described above has been successful with horizontal antenna arrangements as described above. Problems have arisen however in applying this technique to vertical antennas, i.e. antennas which extend in vertical planes and which are horizontally spaced apart. Multiple adjacent passageways utilizing vertical antennas are formed by positioning one pair of spaced apart transmitter and receiver antennas adjacent to a second pair of spaced apart transmitter and receiver antennas. Where only two adjacent passageways are to be provided, one may employ a single continuously operating transmitter antenna and two receiver antennas on opposite sides of and spaced apart from the transmitter antenna to form two adjacent passageways. In such case a single, continuously operating transmitter is used to continuously energize the transmitter antenna. Further, separate receivers and alarms are connected to the two receiver antennas; and multiplexing is used to render only one receiver and alarm active at a time.
While the foregoing prior art arrangements have been suitable for avoiding ambiguities when vertical antennas are used to form two adjacent passageways, problems arise when a third passageway is to be added because in such case the third passageway requires the provision of a second transmitter and a third receiver. Consequently, it becomes necessary to position an antenna of one passageway defining pair at a location immediately adjacent an antenna from another passageway defining pair. Although multiplexing may be used in such case to identify which passage a protected article is carried through other problems arise due to the vertical orientation of the antennas. More particularly it has been found that when antennas of different passageway defining pairs are positioned adjacent each other their circuits cross couple and the inactive antenna imposes a load on the active antenna which severely restricts its effectiveness. It has been proposed to use the multiplexing in a way to disconnect the inactive antenna from its circuit; however this becomes quite expensive and the switching action itself causes transients which could interfere with the system.